I. Field of the Invention
The present invention relates to multiple access communication systems, such as wireless data or telephone systems, and spread spectrum communication systems using satellites. More specifically, the invention relates to a communication system architecture in which communication signals are transmitted using transmission modules employing connected sets of modulators and spreading elements coupled to respective analog transmitters, to decrease data transfer requirements. The invention further relates to a method of redistributing certain signal modulation functions in a CDMA spread spectrum communication system to decrease data transfer rates.
II. Description of the Related Art
A variety of multiple access communication systems and techniques have been developed for transferring information among a large number of system users. The use of spread spectrum modulation techniques, such as code division multiple access (CDMA), in multiple access communication systems is disclosed in the teachings of U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990 under the title xe2x80x9cSpread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeatersxe2x80x9d, and U.S. Pat. No. 5,691,974, which issued Nov. 25, 1997 under the title xe2x80x9cMethod And Apparatus For Using Full Spectrum Transmitted Power In A Spread Spectrum Communication System For Tracking Individual Recipient Phase Time And Energy,xe2x80x9d which are both assigned to the assignee of the present invention, and incorporated herein by reference.
These patents disclose wireless communication systems in which a number of generally mobile or remote system users or subscribers employ transceivers to communicate with other system users or desired signal recipients, such as through a public telephone switching network. The transceivers typically communicate through gateways and satellites, or base stations (also referred to as cell-sites or cells) using code division multiple access (CDMA) spread spectrum type communication signals.
Base stations cover cells, while satellites have footprints on the surface of the Earth. In either system, capacity gains can be achieved by sectoring, or subdividing, the geographical regions being covered. Cells can be divided into xe2x80x9csectorsxe2x80x9d by using directional antennas at the base station. Similarly, a satellite""s footprint can be geographically divided into xe2x80x9cbeamsxe2x80x9d, through the use of beam forming antenna systems. These techniques for subdividing a coverage region can be thought of as creating isolation using relative antenna directionality or space division multiplexing. In addition, provided there is available bandwidth, each of these subdivisions, either sectors or beams, can be assigned multiple CDMA channels through the use of frequency division multiplexing (FDM). In satellite communication systems, each CDMA channel can be referred to as a xe2x80x9csub-beamxe2x80x9d because there may be several of these channels per xe2x80x9cbeamxe2x80x9d, or occupying the area covered by a beam.
In a typical spread-spectrum communication system, one or more, generally a set or pair of, preselected pseudonoise (PN) code sequences are used to modulate or xe2x80x9cspreadxe2x80x9d user information signals over a predetermined spectral band prior to modulation onto a carrier signal for transmission as communication signals. PN spreading, a method of spread-spectrum transmission that is well known in the art, produces a signal for transmission that has a bandwidth much greater than that of the data signal. In the base station- or gateway-to-user communication link, PN spreading codes or binary sequences are used to distinguish between signals transmitted by different base stations or over different beams, as well as between multipath signals. These codes are typically shared by all communication signals within a given CDMA channel or sub-beam.
Orthogonal channelizing codes are used to reduce interference and discriminate between different users within a cell or between user signals transmitted within a satellite sub-beam on a forward link. That is, each user terminal has its own orthogonal channel provided on the forward link by using a unique xe2x80x9ccoveringxe2x80x9d orthogonal code. Walsh functions are generally used to implement channelizing codes, with a typical length being on the order of 64 code chips for terrestrial systems and 128 code chips for satellite systems.
In addition, some form of signal diversity is used to reduce the deleterious effects of fading and additional problems associated with relative user, or satellite, movement within a communication system. Generally, three types of diversity are used in spread spectrum communication systems, including time, frequency, and space diversity. Time diversity is obtainable using error correction coding or simple repetition and time interleaving of signal components. A form of frequency diversity is inherently provided by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space diversity is provided using multiple signal paths, typically, through different antennas or communication signal beams.
Base stations for terrestrial cellular communication systems typically use six antennas, two per each of three sectors in a sub-divided cell. Some designs plan for using additional antennas and polarization modes, providing additional CDMA channels. Base stations used with satellites, also referred to as gateways or hubs, use an array of transmitters, on the order of 32 or more, connected to one or more antennas to accommodate multiple beams on each carrier frequency. also provide service to multiple satellites, typically on the order of three or four at any given time. In one exemplary system, on the order of six satellites are used in each of eight orbital planes and even more satellites are contemplated for some systems. In addition, the number of communication channels or circuits per sub-beam in a satellite is on the order of 128 channels rather than the 64 typically found in terrestrial cellular systems. These factors greatly increase the amount of data and signal processing that must be accommodated within a system gateway as opposed to typical base stations.
When information, including voice, in the form of digital data is to be transferred to system users or subscribers by a gateway it is first encoded and interleaved as desired, and then xe2x80x9ccoveredxe2x80x9d and xe2x80x9cspreadxe2x80x9d using appropriate orthogonal and spreading codes. Each data signal is processed by at least one modulator for each analog signal path over which it is to be transferred, for diversity purposes. The spread encoded data is then transferred to one or more analog transmitters where it is up-converted to an appropriate intermediate frequency and used to modulate a carrier waveform to form a desired communication signal.
Each analog transmitter represents one pre-selected diversity signal path for a signal, and multiple user signals are typically transferred through each analog transmitter, at any time. The signals for each analog transmitter are received from an array or number of modulator elements within the gateway, or base station, which are each assigned to process communications for particular users using particular signal path diversities. The signals from several modulators are combined to form a single output waveform for each analog transmitter. This means that data intended for each analog transmitter must be transferred along common busses or cable assemblies connected to the outputs of all modulators. That is, all modulators and analog transmitters are interconnected or connected using one set of common data busses in order to potentially process multiple path (diversity) signals for any given combination of analog transmitter, antenna, satellite, and user.
For current traffic channel data rates found within communication systems, the gateway busses transferring signals between digital modulators and analog transmitters would have to handle on the order of several gigabits-per-second (Gbps) or more. The output of each modulator provides data at rates on the order of 40 megabits-per-second (Mbps). Potentially, signals for up to 128 user channels can be transmitted on each CDMA channel or frequency using from 2 to 64 diversity paths. This results in total data bus transfer rates in excess of 5-10 Gbps (for example, 40 Mbpsxc3x97128xc3x972). Data transfer rates of this magnitude exceed the current limits of commercially viable bus transfer structures, within reasonable cost and reliability constraints. In addition, cabling and physical interconnection structures for transferring this volume of data among various processing circuit structures also becomes prohibitively complex and potentially unreliable. Higher capacity terrestrial or cellular base stations will also probably have similar processing or data transfer demands in the future.
The control, switching, timing, and so forth, that must be implemented for this data transfer volume is also prohibitively complex for use in cost effective gateway systems. The relative timing of each user signal being transferred to a given analog transmitter must be synchronized to within less than one-half of the chip interval for the spreading code being used, for all other user signals being transferred to the same analog transmitter, in order to be combined for transmission by a common analog transmitter. Such synchronization requires unduly complex and sophisticated control mechanisms, and impacts signal processing flexibility.
Therefore, it is desirable to reduce the quantity of data that must be transferred from one functional element or stage to another within the architecture of a gateway or base station, especially for satellite systems. It is also desirable to make more efficient use of lower cost modular components to provide for easy implementation of complex signal processing structures in a cost effective manner.
In view of the above and other problems found in the art relative to processing communication signals in gateways or base stations in spread spectrum communication systems, one purpose of the present invention is to distribute signal processing requirements for modulating orthogonal channels within communication signals to be transmitted from a gateway or base station.
A second purpose of the invention is to decrease the total volume of data per unit time that is transferred along common signal busses between digital and analog signal processing sections of a gateway in a spread spectrum communication system.
Another purpose is to provide a technique that allows for a more cost effective allocation of processing resources in association with each analog transmitter in a gateway.
One advantage of the invention is that it uses spread spectrum and other digital signal processing modules that are very reliable, easy to manufacture, and cost effective to distribute into parallel arrays for use with gateway analog transmitters.
Other advantages of the invention is that it simplifies data transfer, reduces required bus capacity, and does not require special synchronization of signals that are to be combined into a single analog output. The invention reduces data transfer rates on circuit backplanes, and the number of cables, conductors, or other distribution elements required.
These and other purposes, objects, and advantages are realized in a signal processing architecture for use within a base station in a spread spectrum multiple access communication system, such as code division multiple access (CDMA) type wireless telephone/data communication systems. In these systems, users or system subscribers communicate through base stations or satellites and gateways, using different encoded communication signal channels within given carrier frequencies or CDMA channels. Digital data signals intended for transmission to one or more system users are each transferred to one or more of a plurality of transmission modules, each being associated with a corresponding analog output communication path over which data signals are to be transferred. The number of modules to which each user data signal is transferred is equal to the number of analog communication paths over which it is desired to transfer a given user data signal.
Within each transmission module, the digital data signals are received and encoded, and generally also interleaved, to produce encoded data symbols. The encoded data symbols are then also spread or spread spectrum modulated using at least one preselected pseudorandom noise (PN) spreading code, to form spread communication signals. The outputs for each of a plurality of spread spectrum modulators in each each module are summed together and transferred to a single analog transmitter associated with the transmission module. The analog transmitter forms part of a given analog communication signal output path. This produces a single spread communication signal or channel at a preselected carrier frequency for each module and corresponding analog transmitter.
The transmission modules are defined by or each comprise an encoding and/or interleaving section, and a modulation or channelizing and spreading section. An array of encoders, and when desired corresponding interleavers, form the encoding section, while an array of spread spectrum modulators form the modulation section.
In some embodiments, there are an equivalent number of interleavers and modulators. However, in other embodiments some predetermined degree of time sharing for these elements can be employed. In these configurations, a preselected number of encoders and/or interleavers are used which is less than the total number of user channels to be accommodated by the analog transmitter. The number corresponding spread spectrum modulators is generally larger than the number of encoders and/or interleavers, but may still be less than the total number of users or user channels. Multiplexing of signals may be used in some configurations.
User data or information to be transmitted to one or more users is received and encoded by the encoding section, and the resulting encoded data is processed in the spreading section to generate spread data symbols for each diversity path for each user. The number of processing elements, primarily encoders and spreading elements, within each module is such that at least one processing path is available for each user or user communication channel over which it is desired to transmit information through using a given analog transmitter. The invention allows a large degree of modular construction and circuit integration for purposes of cost reduction and reliability.
In further embodiments of the invention, the encoded data symbols for each analog communication path intended for a given user are covered with one of a plurality of orthogonal functions. The same function is used for each signal transferred on each analog communication path for a given user. Orthogonal code transformers or transformation circuits are also disposed in the transmission modules and work with the spreading sections to perform orthogonal transformations or mappings on encoded data symbols. This generates representative orthogonal user channel data for each user channel operating through a corresponding analog transmitter. Walsh functions are generally used for the orthogonal functions.
In still further embodiments of the invention, the modulation modules spread each of the digital communication signals using preselected in-phase (I) and quadrature (Q) phase pseudorandom noise (PN) code sequences for the particular communication system, with off-sets or time shifts as appropriate. These preselected pseudonoise (PN) sequences are also used to demodulate the in-phase and quadrature signal components when the signals are received by intended recipients.
In this manner, a number of communication or user information signals are transmitted over one or more diversity transfer paths for multiple system users using analog transmitters associated with transmission modules that encode, interleave, and spread the signal, which are transmitted on at least one carrier frequency. The analog transmitters convert digital communication signals to analog communication signals at predetermined sampling rates.
The present invention is very useful for reducing the complexity of signal transfer structures in gateway type base stations which are communicating with at least one satellite based repeater to transfer the communication channel signals to user terminals within the communication system from analog transmitters. This is especially useful where there are at least two satellites in communication with the gateway at any given time.